UC Berkeley

In this work cation, anion, and solvent velocities are reported in concentrated Li-ion battery electrolytes using an electrophoretic NMR (eNMR) methodology. Recent computational studies suggest that in concentrated electrolytes, ion aggregation may lead to transport of Li+ in negatively-charged clusters, resulting in negative cationic transference numbers.1,2 Additionally, increasing ion–solvent coordination may lead to a significant non-zero solvent velocity. These unusual transport properties have important implications for fast-charging performance, but as of yet only limited experimental evidence exists for these phenomena.3 Pulsed-field-gradient (PFG) NMR experiments on their own give self-diffusion coefficients but do not report on the drift of charged species, or induced solvent motion; by running PFG under synchronized application of an electric field (i.e., eNMR), the sign and magnitude of electrophoretic mobilities can be determined with spectroscopic specificity. Through 7Li, 19F and 1H eNMR measurements on carbonate- and glyme-based concentrated electrolytes, we show average species velocities in good agreement with conductivity data, and also directly measure transference numbers in the solvent reference frame. eNMR may provide rapid screening of potential electrolytes at various concentrations to discern which systems (both liquid- and polymer-based) possess efficacious transport properties. (1) France-Lanord, A.; Grossman, J. C. Correlations from Ion Pairing and the Nernst-Einstein Equation. Phys. Rev. Lett. 2019, 122 (13), 136001. https://doi.org/10.1103/PhysRevLett.122.136001. (2) Molinari, N.; Mailoa, J. P.; Kozinsky, B. General Trend of a Negative Li Effective Charge in Ionic Liquid Electrolytes. The Journal of Physical Chemistry Letters 2019, 10 (10), 2313–2319. https://doi.org/10.1021/acs.jpclett.9b00798. (3) Gouverneur, M.; Schmidt, F.; Schönhoff, M. Negative Effective Li Transference Numbers in Li Salt/Ionic Liquid Mixtures: Does Li Drift in the “Wrong” Direction? Physical Chemistry Chemical Physics 2018, 20 (11), 7470–7478. https://doi.org/10.1039/C7CP08580J.